20,362 research outputs found
Role of scattering in virtual source array imaging
We consider imaging in a scattering medium where the illumination goes
through this medium but there is also an auxiliary, passive receiver array that
is near the object to be imaged. Instead of imaging with the source-receiver
array on the far side of the object we image with the data of the passive array
on the near side of the object. The imaging is done with travel time migration
using the cross correlations of the passive array data. We showed in [J.
Garnier and G. Papanicolaou, Inverse Problems {28} (2012), 075002] that if (i)
the source array is infinite, (ii) the scattering medium is modeled by either
an isotropic random medium in the paraxial regime or a randomly layered medium,
and (iii) the medium between the auxiliary array and the object to be imaged is
homogeneous, then imaging with cross correlations completely eliminates the
effects of the random medium. It is as if we imaged with an active array,
instead of a passive one, near the object. The purpose of this paper is to
analyze the resolution of the image when both the source array and the passive
receiver array are finite. We show with a detailed analysis that for isotropic
random media in the paraxial regime, imaging not only is not affected by the
inhomogeneities but the resolution can in fact be enhanced. This is because the
random medium can increase the diversity of the illumination. We also show
analytically that this will not happen in a randomly layered medium, and there
may be some loss of resolution in this case.Comment: 22 pages, 4 figure
Signal to noise ratio analysis in virtual source array imaging
We consider correlation-based imaging of a reflector located on one side of a passive array where
the medium is homogeneous. On the other side of the array the illumination by remote impulsive sources
goes through a strongly scattering medium. It has been shown in [J. Garnier and G. Papanicolaou, Inverse Problems 28 (2012), 075002] that
migrating the cross correlations of the passive array gives an image whose resolution is as good as if
the array was active and the array response matrix was that of a homogeneous medium.
In this paper we study the signal to noise ratio of the image as a function of statistical properties of the
strongly scattering medium, the signal bandwidth and the source and passive receiver array characteristics.
Using a Kronecker model for the strongly scattering medium we show that image resolution is as
expected and that the signal to noise ratio can be computed in an essentially explicit way. We
show with direct numerical simulations using full wave propagation solvers in random media that
the theoretical predictions based on the Kronecker model are accurate
Imaging With Nature: Compressive Imaging Using a Multiply Scattering Medium
The recent theory of compressive sensing leverages upon the structure of
signals to acquire them with much fewer measurements than was previously
thought necessary, and certainly well below the traditional Nyquist-Shannon
sampling rate. However, most implementations developed to take advantage of
this framework revolve around controlling the measurements with carefully
engineered material or acquisition sequences. Instead, we use the natural
randomness of wave propagation through multiply scattering media as an optimal
and instantaneous compressive imaging mechanism. Waves reflected from an object
are detected after propagation through a well-characterized complex medium.
Each local measurement thus contains global information about the object,
yielding a purely analog compressive sensing method. We experimentally
demonstrate the effectiveness of the proposed approach for optical imaging by
using a 300-micrometer thick layer of white paint as the compressive imaging
device. Scattering media are thus promising candidates for designing efficient
and compact compressive imagers.Comment: 17 pages, 8 figure
Ray-optical refraction with confocal lenslet arrays
Two parallel lenslet arrays with focal lengths f1 and f2 that share a common focal plane (that is, which are separated by a distance f1+f2) can refract transmitted light rays according to Snell's law, but with the 'sin's replaced with 'tan's. This is the case for a limited range of input angles and other conditions. Such confocal lenslet arrays can therefore simulate the interface between optical media with different refractive indices, n1 and n2, whereby the ratio η=-f2/f1 plays the role of the refractive-index ratio n2/n1. Suitable choices of focal lengths enable positive and negative refraction. In contrast to Snell's law, which leads to nontrivial geometric imaging by a planar refractive-index interface only for the special case of n1=±n2, the modified refraction law leads to geometric imaging by planar confocal lenslet arrays for any value of η. We illustrate some of the properties of confocal lenslet arrays with images rendered using ray-tracing software
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Towards the identification of spatially resolved mechanical properties in tissues and materials: State of the art, current challenges and opportunities in the field of flow measurements
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.This work is focused on optical methods that provide tomographic reconstructions of the structure
of materials and tissues. Phase information can also be used to measure 3-D displacement and strain fields
with interferometric sensitivity. Different approaches are presented, including recent developments in phase
contrast wavelength scanning interferometry and a combination of optical coherence tomography and digital
volume correlation to estimate elastic properties of synthetic phantoms and porcine corneas. Inversion
algorithms based on finite elements and the Virtual Fields Method (VFM) are used to extract mechanical
properties from the knowledge of the applied loads, geometry and measured deformation fields. Current
efforts into extending these methods into single shot techniques have the potential of expanding the range of
applications to study dynamic events such as micro-flows in engineering and biological systems in which
scattering particles are transported in a flow, e.g. tribology, microfluidic devices, cell migration or multiphase
flows
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